High powered electronic equipment is pushing the frontiers of thermal control technology. In the past, simply blowing air over components has been sufficient to maintain the device temperatures within specified thermal limits. Some applications have used liquid coolant to remove excess heat to a heat sink. Further, thermal radiators, extending from the microelectronic substrates, have been used to improve the thermal cooling efficiency. With the ever increasing speed and decreased size of electronics, simple convective cooling techniques, such as fans, may no longer serve to cool high heat flux components. Microelectronic chips used in home computers currently produce a heat flux on the order of 30 W/cm.sup.2, these levels are expected to increase to 100 W/cm.sup.2 in the very near future.
Circuitry typically fails by a single component subject to temperatures exceeding maximum limits. The reduction in excessive heat extends the life of microelectronic components. It has been estimated that for every 10C drop in hot spot temperature heat exposure, the life of the chip circuitry doubles. Also cracking due to thermal expansion and contraction is a common failure mode. Microcooling devices are employed to distribute the thermal energy uniformly, to reduce hot spot, and limit the amount of thermal expansion and contraction.
One such cooling device is a microthermal cooling heat pipe. An example of a microheat pipe may be found in U.S. Pat. No. 5,309,457, issued May 3, 1994; and Camarda, 5,598,632 issued Feb. 4, 1997. Microthermal cooling heat pipes utilize static wick structures to distribute thermal energy across a substrate. Structurally, the device is a closed vessel, or pipe, of various geometric cross sections with a capillary wick for transporting a working fluid. The wick may be narrow grooves machined into the pipe wall. At one end, the evaporator, heat is added at some temperature, vaporizing the liquid fluid from the wick material. The vaporized fluid flows along the central core of the pipe to a cooler, lower pressure end, known as the condenser. At the condenser end, the vapor is condensed to a liquid with the release of associated latent heat. The condensed liquid is pumped back to the evaporator section by the action of surface tension in the capillary structure of the wick material. The radius of the bottom of the narrow grooves of the capillary wick creates a pumping action as vapor collects at the condenser using incremental pressure gradient along the grooves. Microheat pipes use surface tension to create the pumping action for moving the fluid from a hot spot evaporator to a cold spot condenser in a rigid wick structure. The structure of the heat pipe, requiring tubular pipe formation, is difficult to manufacture in microelectronic and micro-electro-mechanical systems (MEMS). The grooved capillary wicks used in heat pipes have a predetermined bottom radius that limits the effective pumping action. Prior wicks have limited utility in the face of highly variable thermal exposures. These and other disadvantages are solved or reduced using the invention.